The host response to pathogenic insults involves complex inflammatory responses and cellular immune reactions, such that the decision to generate protective immunity or tolerance often depends on the context in which T cells first encounter antigen (Ag). While the induction of immunological tolerance was first described almost 100 years ago, the idea that tolerance could be actively mediated by T cells and transferred to na[unreadable]ve recipients was not proposed until the 1970's. Regulatory T cells (Treg) are critical mediators of immune tolerance, and understanding the mechanisms that govern their generation and function has enormous bearing on the development of therapies for T cell-mediated autoimmune diseases and chronic transplant rejection. There has been a renewed interest in T cell-mediated immune regulation over the last decade, with the vast majority of research focused primarily on CD4+ Treg. However, the early reports detailing immune tolerance described the activity of CD8+ Treg, and there is still very little known regarding how CD8+ Treg develop and function. Studies on T cell tolerance have shown that Ag injection into the anterior chamber (AC) of the eye induces an immune deviation (called anterior chamber associated immune deviation, ACAID) - characterized by the induction of an Ab response, with simultaneous impairment (or tolerance) of cell-mediated immunity. Another key feature of ACAID is that the tolerance is "infectious", i.e. it can be transferred from a tolerant individual to a na[unreadable]ve recipient. In many ways, the immunological response to Ag presented via the AC of the eye is very similar to the response induced after intravenous (i.v.) immunization of soluble Ag: immunological tolerance upon challenge, ability to transfer tolerance to na[unreadable]ve animals with CD8+ T cells obtained from tolerized mice, necessity for the apoptotic death of Ag-coupled cells to induce tolerance, and, based on our published and preliminary data, the key role of TNF-related apoptosis-inducing ligand (TRAIL) in the establishment of tolerance. The tolerance observed in both of these model systems is not simply due to the failure to prime, but results from the generation of CD8+ Treg. To further investigate the hypothesis that the tolerance induced in both of these models is mediated by TRAIL-expressing CD8+ Treg, we propose the following Specific Aims: Aim 1 - Determine the impact of peripheral deletion of antigen-specific T cells on the induction of tolerance by TRAIL-expressing CD8+ Treg, Aim 2 - Determine the mechanism behind the inability of activated apoptotic cells to induce tolerance, compared to na[unreadable]ve apoptotic cells, when injected intravenously, and Aim 3 - Determine the role of TRAIL in the ability of antigen administered through the AC to stimulate antigen-specific regulatory CD8+ Treg that mediate systemic tolerance. As interest in TRAIL has grown since its initial discovery as a tumoricidal molecule, TRAIL is now known to possess functions that go beyond tumor surveillance. There has also been a renewed interest in T cell-mediated suppression with the availability of new reagents and technology. Thus, our proposal will investigate the role of TRAIL-expressing CD8+ Treg in the induction and maintenance of immunological tolerance using 2 classical tolerance models, with the intention of applying our findings in the treatment of autoimmunity or increase transplant survival.